<!DOCTYPE html>

<html lang="en">
    <head>
        <meta charset="UTF-8">
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
        <title>父子元素外边距重叠</title>

        <style type="text/css">
            h3 { text-align: center ; }
            hr { margin: 0 auto ; }
            .wrapper { 
                margin: 25px ;
                /* border: 1px solid blue ; */ 
            }

            .wrapper>div {
                margin: 25px ;
                border: 1px solid transparent ;
                height: 50px;
            }

        </style>

    </head>
    <body>

        <h3>父子元素外边距重叠</h3>

        <hr>

        <div class="wrapper">
            <div></div>
        </div>

        <hr>
        
    </body>
</html>